Spherical carbons and method for preparing the same

ABSTRACT

The present invention provides a method for preparing spherical carbon comprising step of heat-treating a mixture of a carbon precursor and dispersion media, a spherical non-graphitizable carbon using the same, and a method for preparing spherical artificial graphite.

This application is a Divisional of co-pending application Ser. No.11/907,496 filed Oct. 12, 2007. Application Ser. No. 11/907,496 is aDivisional of application Ser. No. 10/297,174 filed on Dec. 3, 2002, nowU.S. Pat. No. 7,297,320, and for which priority is claimed under 35U.S.C. § 120. Application Ser. No. 10/297,174 is the national phase ofPCT International Application No. PCT/KR02/00707 filed on Apr. 17, 2002under 35 U.S.C. § 371 and claims priority to Application No.2001-0020462 filed in Korea on Apr. 17, 2001 and Application No.2001-0056846 filed in Korea on Sep. 14, 2001. The entire contents ofeach of the above-identified applications are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to spherical carbon, and particularly tospherical carbon with a size of several to several tens of micrometerthat can be used for anode active material for a lithium secondarybattery. The present invention also relates to a method for preparingthe spherical carbon.

(b) Description of the Related Art

As anode active material for a lithium secondary battery, graphitematerial such as natural graphite and artificial graphite,non-graphitizable carbon or hard carbon, graphitizable carbon or softcarbon, etc. are used. Graphitized g-MCMB (Graphitized MesoCarbonMicroBeads, product of Japan Osaka Gas Chemical Co.), which is one typeof artificial graphite, is used the most.

The reasons why g-MCMB is preferred are that a battery using thematerial has a high energy density per battery volume because the sizeof carbon particles is several to several tens of micrometers and thusg-MCMB can achieve a high rate of packing in a battery, and a batteryemploying g-MCMB has a small initial irreversible capacity because thecarbon particles are spherical such that they have a small specificsurface area. Initial irreversible capacity refers to the chargerequired for forming a passivation film while electrolyte is decomposedon a surface carbon when initially charging a lithium secondary battery,and the required packing is a factor in limiting battery capacitybecause it cannot be used in a subsequent discharge process. Such filmforming is an unavoidable process when using carbon material as ananode. It is therefore important to minimize film forming, which ispossible by minimizing the specific surface area of carbon that is usedas an anode material.

Non-graphitizable carbon is produced by carbonizing a resin precursor at700 to 1500° C. under inert atmosphere. Hence, non-graphitizable carbonhas a low unit cost of production compared to artificial graphiterequiring a high temperature heat treatment of 2500° C. or more.Further, non-graphitizable carbon has a reversible capacity of 400 mAh/gor more compared to graphite carbon (natural carbon, artificial carbonsuch as g-MCMB), which has a reversible capacity of approximately 300mAh/g (the theoretical reversible capacity is 372 mAh/g).

Practically, there are two reasons why non-graphitizable cannot bewidely used for a battery.

First, the crystallinity of non-graphitizable carbon is not high andnon-graphitizable carbon includes fine pores and thus has a low density,while graphite carbon has a crystalline structure with a highcrystallinity and well-developed graphite layers and thus it has a highdensity. Since the non-graphitizable carbon has a low density, thevolume of an anode becomes large if it is packed in a battery such thatthe energy density per battery volume becomes low. Specifically,assuming that carbon of the same weight is packed, non-graphitizablecarbon occupies more volume than graphite carbon.

Second, general non-graphitizable carbon must undergo a pulverizingprocess in order to be used for a battery because it is produced in amassive form. Particles of pulverized carbon have an irregular shape anda large specific surface area. A packing density becomes low because theshapes of the particles are irregular, and, because of the largespecific surface area, an initial irreversible capacity becomes largesuch that an initial coulomb efficiency is lowered.

Accordingly, in order to take advantage of the inexpensive cost and highreversible capacity properties of non-graphitizable carbon, a method formaking particles of the carbon spherical requires examination.Specifically, if the particles of non-graphitizable carbon arespherical, a tap density will be high, thereby allowing a large amountof the carbon to be packed. Also, the specific surface area of thecarbon will be small and thus an initial irreversible capacity candecrease. If non-graphitizable carbon can be produced in spherical form,the problems of non-spherical non-graphitizable carbon, i.e., a lowpacking density and large initial irreversible capacity, can besimultaneously solved.

Graphitizable carbon refers to carbon that becomes artificial carbonwhen heat-treated at a high temperature of 2500° C. or more aftercarbonizing a pitch precursor at 700 to 1500° C. under inert atmosphere.As spherical artificial carbon, g-MCMB is widely used, which is preparedby heat-treating pitch at 300 to 500° C. to make mesophase spherulite,then by performing the processes of cooling, extracting with solvent,carbonizing and graphitizing. However, this process has a low yield andthe production cost is high.

SUMMARY OF THE INVENTION

The present invention is made in consideration of the problems of theprior art. It is an object of the present invention to provide sphericalcarbon having a high tap density and a small specific surface area, andto provide a method for preparing the same.

It is another object of the present invention to provide sphericalcarbon and a method for preparing the same, in which the sphericalcarbon can increase a packing density when used as anode active materialfor a lithium secondary battery and thus increase battery capacity perunit volume, and has a small specific surface area and thus decrease aninitial irreversible capacity.

It is yet another object of the present invention to providenon-graphitizable spherical carbon that can be prepared by a simplermethod than that used for the existing non-graphitizable carbon, and amethod for preparing the same.

It is still yet another object of the present invention to providegraphitizable spherical carbon that can be prepared by a simpler methodthan that used for the existing graphitizable carbon, and a method forpreparing the same.

It is still yet another object of the present invention to provide amethod for preparing spherical artificial graphite that can be preparedby a simpler method than that used for the existing artificial graphite.

It is still yet another object of the present invention to provide amethod for preparing a spherical carbon precursor that can be used forvarious uses.

In order to achieve these objects, the present invention provides amethod for preparing spherical carbon comprising the step ofheat-treating a mixture of a carbon precursor and a dispersion media.

The present invention also provides a method for preparing sphericalartificial graphite comprising the steps of

a) heat-treating a mixture of a carbon precursor and a dispersion media;and

b) heat-treating a spherical carbon precursor or spherical carbonprepared by heat-treating at 2000 to 3200° C.

The present invention also provides spherical non-graphitizable carboncomprising 10 wt % or more of spherical particles satisfying thefollowing Mathematical Formula 1:

0.99≦a/b≦1  [Mathematical Formula 1]

where a is the minor axis of a particle, and b is the major axis of aparticle.

The present invention also provides a battery comprising the sphericalcarbon or spherical artificial graphite as anode material.

The present invention also provides a method for preparing a sphericalcarbon precursor comprising the step of heat-treating a mixture of acarbon precursor and a dispersion media at a glass transitiontemperature or a softening temperature of the carbon precursor to 600°C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of the spherical carbon of Example 1 by ScanningElectron Microscopy with a magnifying power of 1000.

FIG. 2 is a photograph of the spherical carbon of Example 8 by ScanningElectron Microscopy with a magnifying power 1000.

FIG. 3 is a photograph of the spherical carbon of Example 9 by ScanningElectron Microscopy with a magnifying power 1000.

FIG. 4 is a particle size distribution chart of the spherical carbon ofExample 9.

FIG. 5 is a first and second discharge curve graph of the battery ofExample 11.

FIG. 6 is a graph showing a charge capacity, a discharge capacity and acoulomb efficiency by cycles of the battery of Example 11.

FIG. 7 is a photograph of the spherical artificial graphite of Example12 by Scanning Electron Microscopy with a magnifying power of 2500.

FIG. 8 is a one-time charge/discharge graph of a lithium secondarybattery using the spherical artificial graphite of Example 17 as anodeactive material.

FIG. 9 is a ten times discharge capacity graph of a lithium secondarybattery using the spherical artificial graphite of Example 17 as anodeactive material.

FIG. 10 is a photograph of the spherical carbon precursor of Example 18by Scanning Electron Microscopy with a magnifying power of 1000.

DETAILED DESCRIPTION AND THE PREFERRED EMBODIMENTS

The present invention will now be explained in detail.

The present invention provides spherical carbon and spherical artificialgraphite that can be used as anode active material for a lithiumsecondary battery, and a method for preparing the same. The presentinvention adds a dispersion media of silicone oil or an inorganicsubstance, a surface of which is treated so as to have hydrophobicity,to a carbon precursor resin, pitch, etc. to mix these substances, thenheat-treats the mixture to make the carbon precursor spherical.

If a carbon precursor is directly carbonized or graphitized, carbon of amassive form will be obtained. Accordingly, it is necessary to perform apulverization process in order to enable use as electrode material.Pulverized carbon particles have non-spherical irregular shapes, andthus have a low packing density and a large specific surface areacompared to spherical carbon.

However, the hydrophobic inorganic substance or silicone oil dispersionmedia of the present invention makes a carbon precursor convert intospherical carbon during heat-treating process.

Specifically, when the carbon precursor is composed of irregularanisotropic pitch particles, if the pitch is mixed with the hydrophobicinorganic substance or silicone oil and heat-treated at a softeningtemperature of the pitch or more, compounds comprising the pitch becomefluid and convert into mesophase. At this time, mesophase pitchparticles tend to cohere to each other, but the mixed hydrophobicinorganic substance or silicone oil blocks this process of the pitchparticles to restrain their cohesion. Therefore, since each mesophasepitch particle that is separately dispersed has a tendency of minimizingsurface energy, the carbon precursor converts into a spherical form thathas the lowest surface energy.

Further, when the carbon precursor is a resin, if heat-treated at atemperature above the glass transition temperature of the resin, thepolymer chain becomes more fluid, and since the hydrophobic inorganicsubstance or silicone oil restrains cohesion between resin particles,resin particles convert into spherical form in order to minimize surfaceenergy.

Accordingly, the hydrophobic inorganic substance or silicone oil of thepresent invention is distributed on the surface of carbon precursorparticles to restrain the cohesion of precursor particles duringheat-treatment, and, when the precursor particles contract, provides ahigh surface tension to make the particles convert into spherical form.After such heat-treatment, a dispersion media is removed to obtain aspherical carbon precursor or spherical carbon with a high purity.Whether a spherical carbon precursor or spherical carbon is obtained isdetermined by a temperature for heat-treating the non-spherical carbonprecursor together with a dispersion media. Also, spherical artificialgraphite can be obtained by heat-treating the spherical carbon precursoror spherical carbon at a high temperature.

The spherical carbon of the present invention has a high tap density andthus when used as anode active material for a lithium secondary battery,a large quantity thereof can be packed. Also, the spherical carbon ofthe present invention has a small specific surface area such that itsinitial irreversible capacity becomes low (a high initial coulombefficiency).

The spherical carbon particles can be classified into three types: oneis non-graphitizable spherical carbon prepared using resin or isotropicpitch as a carbon precursor, a second is graphitizable spherical carbonprepared using anisotropic pitch as a carbon precursor, and the third isspherical artificial graphite prepared by heat-treating thegraphitizable spherical carbon at a high temperature. Particularly,spherical artificial graphite can be easily prepared by heat-treating adispersion media-removed spherical carbon precursor or spherical carbonat 2500° C. or more, preferably 2000 to 3200° C.

Among the different types of spherical carbon, sphericalnon-graphitizable carbon has not been able to be prepared until now. Thespherical non-graphitizable carbon comprises at least 10 wt % ofparticles having a minor-axis-to-major-axis ratio of 0.99 to 1(satisfying the above Mathematical Formula 1). The remaining particlesmay have a minor-axis-to-major-axis ratio of 0.1 to 0.99 (satisfying theMathematical Formula 2 below):

0.1≦a/b<0.99  [Mathematical Formula 2]

-   -   where a is the minor axis of a particle, and b is the major axis        of a particle.

In addition, the average diameter of the particles is 1 to 40 μm and aspecific surface area is 3 m²/g or less.

Thus prepared non-graphitizable carbon, if used as anode active materialfor a lithium secondary battery, has a high packing density and a lowinitial irreversible capacity (a high initial coulomb efficiency)compared to non-spherical non-graphitizable carbon. In practice, whenthe spherical non-graphitizable carbon of the present invention is usedas anode active material to comprise half of a battery as shown inExample 11, a long flat curve portion appears between 0.00 to 0.2 V (forLi/Li+) in a discharge curve (when lithium gets out of carbon material).

According to the present invention, spherical graphitizable carbon canalso be prepared. If the spherical graphitizable carbon is graphitizedto prepare artificial graphite, the preparation process is simpler andthe yield is higher than g-MCMB. The graphitizable carbon, andartificial carbon prepared therefrom show a particle distribution, adiameter, and a specific surface area similarly to the sphericalnon-graphitizable carbon.

The method for preparing spherical carbon of the present invention willnow be explained.

According to the present invention, the spherical carbon is prepared bythe following two methods.

That first method comprises

a) heat-treating a mixture of a carbon precursor and a hydrophobicinorganic dispersion media at 700 to 1500° C. under inert atmosphere tocarbonize the mixture; and

b) adding an acid or alkali solvent to the a) carbide to removeinorganic substance.

The second method comprises

a) heat-treating a mixture of a carbon precursor and a silicone oildispersion media at a glass transition temperature or a softeningtemperature of the carbon precursor to 300° C. to make the carbonprecursor spherical;

b) adding an organic solvent to the a) spherical carbon precursor toremove silicone oil; and

c) heat-treating the b) spherical carbon precursor at 700 to 1500° C.under inert atmosphere to carbonize the spherical carbon precursor.

As the carbon precursors used for preparing spherical carbon, those ofsolid powder form that can be mixed with a dispersion media, i.e., ahydrophobic inorganic substance or silicone oil, can be used. Therefore,the kinds of the precursors are not specifically limited, and resin,pitch or a mixture thereof can be used according to need. For preparingspherical non-graphitizable carbon, resin or isotropic pitch ispreferably used as the carbon precursor.

The isotropic pitch includes isotropic petroleum pitch or isotropic coaltar pitch, etc., and these are used after oxidation. As the resin,thermosetting synthetic resin is preferable. The thermosetting syntheticresin is selected from the group consisting of phenolic resins, furanresin, epoxy resin, polyacrylonitrile resin, polyimide resin,polybenzimidazole resin, polyphenylene resin, biphenol resin,divinylbenzene styrene copolymer, cellulose and a mixture thereof.

In addition, for preparing spherical graphitizable carbon, pitch ispreferably used as the carbon precursor. The pitch may be petroleumpitch or coal tar pitch, and pitch derived from naphthalene ormethylnaphthalene can be used.

For preparing spherical carbon of the present invention, a hydrophobicinorganic substance or silicone oil dispersion media is added to thecarbon precursor and heat-treated. If the carbon precursor isheat-treated without introducing a hydrophobic inorganic substance orsilicone oil dispersion media, non-spherical mass carbon will beobtained. However, if a hydrophobic inorganic substance or silicone oildispersion media is added to the carbon precursor then heat-treated,spherical carbon can be obtained.

As the hydrophobic inorganic substance used as a dispersion media, thosehaving a hydrophobic surface can be used Silica, zeolite, alumina,titania (TiO₂), ceria (CeO₂), etc., surface of which are hydrophobicallytreated, are examples that can be used. Other kinds of inorganicsubstances can be used if appropriate for the present invention.Particularly, silica is preferable because it can be easily dissolved inweak acid or a weak alkali solution and removed, and because it is lowcost and has a small particle size.

Silica surface-treated with a hydrophobic substance includes CAB-O-SILTS-720, TS-610, TS-530, TS-500, TG-308F, TG-810G530, etc. from CabotCompany; and AEROSIL R972, R974, R812, R812S, R202, etc. from DeggusaCompany. As the inorganic substance, a commercial product can be used oran inorganic substance can be made hydrophobic.

For making inorganic substance hydrophobic, it is preferable to add aninorganic substance, which has a non-hydrophobic surface, to a solventsuch as toluene together with an organosilane surface-treating agentsuch as trimethylchlorosilane, and to reflux the mixture while agitatingthe same to prepare an inorganic substance having a hydrophobic surface.

The mixing ratio of the carbon precursor and the hydrophobic inorganicsubstance is preferably 100:0.1 to 1000 by weight ratio. If the contentsof the hydrophobic inorganic substance are less than 0.1 weight partsper 100 weight parts of the carbon precursor, spherical carbon isdifficult to prepare, and if more than 1000 weight parts, the effectscorresponding to the contents cannot be obtained (i.e., a directrelation between contents and effects does not result).

As the silicone oil used for a dispersion media, silicone oil that doesnot dissolve carbon precursor resin or pitch and does not havereactivity must be used. Further, it is preferable to select siliconeoil that has a higher specific gravity than resin or pitch such thatresin or pitch can be effectively dispersed, and so that cohesion can berestrained. If silicone oil having a lower specific gravity than resinor pitch is used, it is preferable to reduce a corresponding amount ofresin or pitch to restrain cohesion between particles. In addition,sinking of resin or pitch particles to the bottom of a reactor andcohesion thereof can be restrained through agitation.

The mixing ratio of the carbon precursor and the silicone oil ispreferably 100:0.1 to 100000 by volume ratio. If the contents of thesilicone oil are less than 0.1 volume parts per 100 volume parts ofcarbon precursor, spherical carbon is difficult to prepare, and if morethan 100000 volume parts, effects corresponding to the contents cannotbe obtained (i.e., a direct relation between contents and effects doesnot result).

The dispersion media of the present invention is removed after producinga spherical carbon precursor or spherical carbon. The dispersion mediacan be removed by adding solvent to dissolve it. The solvent is selectedon the basis of how well it dissolves dispersion media while notdissolving resin or pitch.

If hydrophobic inorganic substance is used for the dispersion media, anacid or alkali solution is selected for use as a removing solvent. Ifhydrophobic silica is selected for the hydrophobic inorganic substance,hydrofluoric acid solution or alkali solution, etc. can be used as aremoving solution. For example, if hydrofluoric acid is used, sphericalcarbon covered with silica is agitated in 20 to 50 wt % of ahydrofluoric acid solution at room temperature for 30 minutes to 48hours to dissolve the silica, thereby removing the same.

Further, if silicone oil is used as the dispersion media, an organicsolvent is selected as a removing solvent. The organic solvent ispreferably alcohol, and more preferably ethyl alcohol.

According to the present invention, for preparing spherical carbon, acarbon precursor is carbonized. The first method uses a hydrophobicinorganic substance as a dispersion media, and thus a carbon precursoris carbonized while directly made spherical during the carbonizingprocess. The carbonization is conducted by heat-treating a mixture of acarbon precursor and a hydrophobic inorganic substance at 700 to 1500°C. under inert atmosphere (for example, argon, nitrogen, helium, etc,),and spherical carbon is directly prepared during the heat-treatingprocess. Heat-treating is preferably conducted for 1 minute to 50 hours,and a speed for elevating the temperature to a heat-treating temperatureis preferably 0.1 to 100° C./min.

The second method uses silicone oil as a dispersion media, and thus amixture of a carbon precursor and silicone oil is primarily heat-treatedat a temperature lower than a carbonization temperature of the carbonprecursor to make the carbon precursor spherical, and silicone oil isremoved and then the carbon precursor is secondly heat-treated at atemperature higher than the carbonization temperature of the carbonprecursor.

Since most of the silicone oil is decomposed at 300° C. or higher duringthe first heat-treatment, if resin is used as a carbon precursor,heat-treatment is conducted at a temperature more than a glasstransition temperature of the resin less than 300° C., and if pitch isused as a carbon precursor, heat-treatment is conducted at a temperaturemore than a softening temperature of the pitch less than 300° C. Ifhydrophobic inorganic substance is used as a dispersion media instead ofsilicone oil in the second method, a temperature for making the carbonprecursor spherical may be up to 600° C., at which point the carbonprecursor is not carbonized.

The spherical carbon precursor prepared using silicone oil as adispersion media is secondly heat-treated under the same carbonizationconditions as using a hydrophobic inorganic substance dispersion media.In order to prepare spherical carbon that maintains a spherical form anddoes not have cracks, it is preferable to perform oxidativestabilization a carbon precursor at 100 to 400° C. for 1 minute to 2hours under air atmosphere. Such an oxidative stabilization step is morepreferable for preparing artificial graphite.

The method for preparing spherical artificial graphite of the presentinvention will now be explained.

The spherical graphite of the present invention is prepared byheat-treating the above-explained spherical graphitizable carbon at ahigh temperature to graphitize the carbon, or by directly heat-treatinga spherical carbon precursor at a high temperature to graphitize theprecursor.

More specifically, the spherical graphite can be prepared by thefollowing 5 methods.

The first method comprises

a) heat-treating a mixture of a carbon precursor and a hydrophobicinorganic substance dispersion media at 700 to 1500° C. under inertatmosphere to carbonize the precursor;

b) adding an acid or alkali solvent to the a) carbide to remove theinorganic substance; and

c) heat-treating the b) spherical carbon at 2000 to 3200° C.

The second method comprises

a) heat-treating a mixture of a carbon precursor and a hydrophobicinorganic substance dispersion media at a softening temperature of thecarbon precursor to 600° C. to make the carbon precursor spherical;

b) adding an acid or alkali solvent to the a) spherical carbon precursorto remove the inorganic substance;

c) performing oxidative stabilization of the b) spherical carbonprecursor at 100 to 400° C. for 1 minute to 2 hours under airatmosphere;

d) heat-treating the c) spherical carbon precursor at 700 to 1500° C.under inert atmosphere to carbonize the precursor; and

e) heat-treating the d) spherical carbon at 2000 to 3200° C.

The third method comprises

a) heat-treating a mixture of a carbon precursor and a hydrophobicinorganic substance dispersion media at a softening temperature of thecarbon precursor to 600° C. to make the carbon precursor spherical;

b) adding an acid or alkali solvent to the a) spherical carbon precursorto the remove inorganic substance;

c) performing oxidative stabilization of the b) spherical carbonprecursor at 100 to 400° C. for 1 minute to 2 hours under airatmosphere; and

d) heat-treating the c) spherical carbon at 2000 to 3200° C.

The fourth method comprises

a) heat-treating a mixture of a carbon precursor and a silicone coildispersion media at a softening temperature of the carbon precursor to300° C. to make the carbon precursor spherical;

b) adding an organic solvent to the a) spherical carbon precursor toremove the silicone oil;

c) performing oxidative stabilization of the b) spherical carbonprecursor at 100 to 400° C. for 1 minute to 2 hours under airatmosphere;

d) heat-treating the c) spherical carbon precursor at 700 to 1500° C.under inert atmosphere to carbonize the precursor; and

e) heat-treating the d) spherical carbon at 2000 to 3200° C.

The fifth method comprises

a) heat-treating a mixture of a carbon precursor and a silicone oildispersion media at a softening temperature of the carbon precursor to300° C. to make the carbon precursor spherical;

b) adding an organic solvent to the b) spherical carbon precursor toremove the silicone oil;

c) performing oxidative stabilization of the b) spherical carbonprecursor at 100 to 400° C. for 1 minute to 2 hours under airatmosphere; and

d) heat-treating the c) spherical carbon precursor at 2000 to 3200° C.

These methods for preparing artificial graphite use the above-explainedmethods for preparing spherical carbon and a spherical carbon precursor.In addition to the above 5 methods, various other methods can be used toprepare spherical artificial graphite by varying the selection of acarbon precursor and a dispersion media.

In addition, the present invention provides a method for preparingspherical carbon comprising the step of heat-treating a mixture of acarbon precursor and a dispersion media at a glass transitiontemperature of the carbon precursor to 600° C. The temperature of 600°C. is a temperature at which a carbon precursor does not convert intocarbon and can exist as a carbon precursor. Thus the prepared sphericalcarbon precursor comprises 10 wt % or more of spherical particlessatisfying the above Mathematical Formula 1 identically as with thespherical carbon. The spherical carbon precursor can be used for varioususes such as a material for a Braun tube.

The spherical non-graphitizable carbon and spherical artificial graphiteof the present invention, which comprise 10 wt % or more of sphericalparticles satisfying the above Mathematical Formula 1, are suitable foranode active material for a battery such as a lithium secondary battery.

In order to use the spherical carbon of the present invention as anodeactive material for a lithium secondary battery, an electrode is formed.For example, the spherical carbon prepared according to the above methodand a binder are added to a dispersion media at a weight ratio of 10:0.1to 2 and agitated to prepare a paste, and then the paste is coated on ametal material used as a current collector, compressed and dried toprepare an electrode of a laminate shape.

Representative examples of the binder include polytetrafluoroethylene(PTFE), polyvinylidene fluoride (PVdF), and cellulose, and examples ofthe dispersion media include isopropyl alcohol, N-methylpyrrolidone(NMP), and acetone.

As the metal material used as a current collector, any metal that has ahigh conductivity and to which the paste can be easily adhered can beused. Representative examples include mesh and foil comprised of copperor nickel.

A method for uniformly coating the metal material using a paste ofelectrode material can be selected from known methods or conducted by anew appropriate method in consideration of the properties of thematerial. One example is to distribute a paste on a current collectorand uniformly dispersing the paste using a doctor blade, etc. Dependingon the circumstances, the distribution and dispersion processes can beconducted in one process. Additional examples include die-casting, commacoating, and screen-printing, or a process by which an electrode isformed on a separate substrate and then joined to a current collector bypressing or using a lamination method.

A method for drying a coated paste includes drying in a vacuum oven setat 50 to 200° C. for 1 to 3 days. Depending on the circumstances, inorder to further reduce a resistance of an electrode, 0.1 to 20 wt % ofcarbon black can be added as a conducting material. Commercialconducting material includes acetylene black (product of ChevronChemical Company or Gulf Oil Company), Keyjenblack EC (product of ArmakCompany product), Vulcan XC-72 (product of Cabot Company), and Super P(product of MMM Company).

In an example to construct a lithium secondary battery using theelectrode prepared according to the above method, the electrode is usedas an anode and LiCoO₂, LiNiO₂, LiMn₂O₄, etc. are used as a cathode, anda separator film is inserted therebetween. The separator film functionsto block the internal short-circuit of two electrodes and to impregnatean electrolyte. Polymer, a glass fiber mat, and kraft paper can be usedas the separator film. Examples of commercially available productsinclude Celgard 2400, 2300 (product of Hoechest Celanese Corp.) andpolypropylene membrane (product of Ube Industries Ltd. or Pall RAICompany).

The electrolyte is a system dissolving lithium salts in an organicsolvent, and as the lithium salts, LiClO₄, LiCF₃SO₂, LiAsF₆, LiBF₄,LiN(CH₃SO₂)₂, LiPF₆, LiSCN and LiC(CF₃SO₂)₃, etc. can be used, and asthe organic solvent, ethylene carbonate, propylene carbonate, diethylcarbonate, dimethylcarbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane,gamma-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran,1,3-dioxolane, 4-methyl-1,3-dioxoalne, diethyl ether, sulfolane, and amixture thereof can be used.

The present invention will be explained in more detail with reference tothe following Examples. However, the Examples act merely to illustratethe present invention and are in no way do they limit the presentinvention.

EXAMPLES Example 1 Preparation of Spherical Non-Graphitizable CarbonUsing Resin as a Precursor and a Hydrophobic Inorganic Substance as aDispersion Media

After mixing a phenolic resin precursor with CAB-O-SIL TS-530 fumedsilica, a surface of which is treated with hexamethyldisilazane, at aweight ratio of 10:0.5, the mixture was heated at atemperature-elevation speed of 10° C./min. under argon gas atmosphereand heat-treated at 1000° C. for 1 hour to obtain spherical carbon. Theprepared spherical carbon showed 50 to 55 wt % of carbonization yield.FIG. 1 is a Scanning Electron Microscopy photograph of the preparedcarbon.

Example 2 Preparation of Spherical Non-Graphitizable Carbon Using Resinas a Precursor and a Hydrophobic Inorganic Substance as a DispersionMedia

Spherical carbon was prepared by the same method as in Example 1, exceptthat there was used CAB-O-SIL TS-720 fumed silica, a surface of which istreated with polydimethylsiloxane.

Example 3 Preparation of Spherical Non-Graphitizable Carbon Using Resinas a Precursor and a Hydrophobic Inorganic Substance as a DispersionMedia

Spherical carbon was prepared by the same method as in Example 1, exceptthat CAB-O-SIL TS-530 fumed silica, a surface of which is treated withhexamethyldisilazane, was added in the amount of 2 wt % of theprecursor.

Example 4 Preparation of Spherical Non-Graphitizable Carbon Using Resinas a Precursor and a Hydrophobic Inorganic Substance as a DispersionMedia

Spherical carbon was prepared by the same method as in Example 1, exceptthat there was used 200 wt % of zeolite, a surface of which was madehydrophobic by refluxing zeolite Y and trimethylchlorosilane in atoluene solution for 16 hours while agitating the mixture.

Example 5 Preparation of Spherical Non-Graphitizable Carbon Using Resinas a Precursor and a Hydrophobic Inorganic Substance as a DispersionMedia

Spherical carbon was prepared by the same method as in Example 1, exceptthat heat-treating was performed at 700° C. for 1 hour.

Example 6 Preparation of Spherical Graphitizable Carbon Using Pitch as aPrecursor and a Hydrophobic Inorganic Substance as a Dispersion Media

Spherical carbon was prepared by the same method as in Example 1, exceptthat naphthalene isotropic pitch was used as a precursor.

Example 7 Preparation of Spherical Carbon Using a Mixture of Resin andPitch as a Precursor and a Hydrophobic Inorganic Substance as aDispersion Media

Spherical carbon was prepared by the same method as in Example 1, exceptthat there was used a mixture of phenolic resin and naphthaleneisotropic pitch as a precursor.

Example 8

Material of Example 6 was mixed in the same ratio, the mixture washeat-treated to 400° C. for 1 hour under argon gas atmosphere, and thenheat-treated at 270° C. for 30 minutes under air atmosphere. The mixturewas heat-treated again at 1000° C. for 1 hour under argon atmosphere toobtain spherical graphitizable carbon. FIG. 2 is a Scanning ElectronMicroscopy photograph of the prepared carbon.

Example 9

After introducing the spherical carbon prepared in Example 1 and a 48%hydrofluoric acid solution into a reaction vessel made from Teflon andremoving an inorganic substance for 2 days, the mixture was washed withultra-pure water and dried in a vacuum oven set at 120° C. for more than12 hours.

FIG. 3 is a Scanning Electron Microscopy photograph of the preparedcarbon, and FIG. 4 is a particle size distribution chart. A tap densityof the carbon is 0.87 g/cm³ as measured by the ASTM Standard 527-93method, and the specific surface area is 1.7 m²/g as measured by the BETmethod. For reference, the particle size of g-MCMB presently used asanode material for a lithium secondary battery is 6 to 25 μm and the BETspecific surface area of g-MCMB is 1.5 to 3.3 m²/g.

Example 10 Preparation of Artificial Graphite

After introducing graphitizable carbon prepared in Example 6 and a 48%hydrofluoric acid solution in a reaction vessel made from Teflon andremoving an inorganic substance, the mixture was washed with ultra-purewater and dried in a vacuum oven set at 120° C. for more than 12 hours.

The dried carbon was introduced in a furnace set at 2800° C. andheat-treated for 2 hours to prepare spherical artificial graphite.

Example 11 Lithium Secondary Battery Using Spherical Carbon as anElectrode Active Material

Spherical carbon prepared in Example 9 as an active material andpolytetrafluoroethylene (PTFE) as a binder were mixed at a weight ratioof 10:0.5 to prepare a paste. The paste was adhered to a copper meshcurrent collector to prepare a working electrode, and the electrode wasdried in a vacuum oven set at 120° C. for more than 12 hours.

LiCoO₂ was used as a counter electrode, lithium metal foil was used as areference electrode, and 1 mole of LiPF₆/EC:DEC (volume ratio 1:1) wasused as an electrolyte to prepare a beaker-shaped three electrode cellin a dry box under argon atmosphere, and a constant current constantvoltage test for the cell was conducted at room temperature under thefollowing conditions. Constant current was added to 0 V (vs. Li/Li+) ata current density of 20 mAg⁻¹ and constant voltage was added to 0 V tocharge the cell until the current density became less than 1 mAg⁻¹, anddischarge was conducted to 3 V (vs. Li/Li+) at a current density of 20mAg⁻¹. A 5-minute open time was set between charge/discharge.

FIGS. 5 and 6 are graphs showing results of the test conducted under theabove conditions. When charge/discharge was conducted 10 times, areversible capacity of 450 mAh/g appeared and an initial coulombefficiency increased to 68%, compared to 50 to 60% of an initial coulombefficiency for the general non-graphitizable carbon.

Example 12 Preparation of Spherical Artificial Graphite

As a precursor, methylnaphthalene-derived anisotropic pitch (softeningtemperature=227° C.) was pulverized to several to several tens ofmicrometers to disperse the resulting particles in a silicone oildispersion media The particles were then heat-treated at 300° C. for 1hour under argon atmosphere.

The mixture of the pitch and silicone oil was filtered and silicone oilremaining on the surface of the pitch was washed with ethanol to removethe remaining silicone oil.

The spherical pitch underwent oxidative stabilization at 270° C. for 10minutes under air atmosphere, and then was carbonized at 1000° C. for 1hour under argon atmosphere to realize conversion into sphericalgraphitizable carbon.

Next, the graphitizable carbon was heat-treated at 3000° C. for 30minutes under argon atmosphere to graphitize the carbon.

The Scanning Electron Microscopy photograph of the spherical artificialgraphite obtained by the above method is shown in FIG. 7. Table 1 showsa tap density and a specific surface area of the synthesized artificialgraphite. Compared to graphitized MCMB 10-28 (g-MCMB, GraphitizedMesocarbon Microbeads, product of Japan Osaka Gas Chemicals Co., averageparticle size is 10 μm and graphitized at 2800° C.), it can be seen thatthe specific surface area is small and the tap density is high.

Example 13

Spherical anisotropic pitch having undergone oxidative stabilization inExample 12 was directly heat-treated to 3000° C. for 30 minutes underargon atmosphere (without undergoing carbonization at 1000° C.) tographitize the pitch to prepare spherical artificial graphite.

Example 14

Phenolic resin (glass transition temperature=85° C.) was pulverized toseveral to several tens of micrometers to disperse the resin in siliconeoil, and then heat-treated at 100° C. for 1 hour under argon atmosphere.

After filtering a mixture of spherical resin particles and silicone oil,remaining resin particles were washed with ethanol to remove thesilicone oil.

Subsequently, the mixture was heat-treated at 1000° C. for 1 hour underargon atmosphere to realize conversion into spherical non-graphitizablecarbon.

Table 1 shows the specific surface area and tap density of thesynthesized non-graphitizable carbon.

TABLE 1 Comparison of g-MCMB 10-28, spherical artificial graphite andspherical non-graphitizable carbon Specific surface area Tap densityCarbon type (m²/g) (g/cm³) g-MCMB 10-28 2.5 1.41 Spherical artificial1.9 1.61 graphite (Example 12) Spherical non- 1.8 0.86 graphitizablecarbon (Example 14)

In Table 1, the specific surface areas are obtained from nitrogenadsorption isothem and the tap densities are measured by ASTM No.B527-93.

Example 15

Spherical carbon was prepared by the same method as in Example 14,except that naphthalene isotropic pitch (softening temperature=165° C.)was used as a precursor and heat-treatment was conducted at 180° C. for1 hour.

Example 16

Spherical carbon was prepared by the same method as in Example 14,except that a mixture of phenolic resin and naphthalene isotropic pitchwas used as a precursor and heat-treatment was conducted at 180° C. for1 hour.

Example 17 Lithium Secondary Battery Using Spherical Carbon as anElectrode Active Material

The spherical artificial graphite prepared in Example 12 as activematerial and polytetrafluoroethylene (PTFE) as a binder were mixed at aweight ratio of 10:0.5 to prepare a paste, the paste was adhered to acopper mesh current collector to prepare a working electrode, and theelectrode was dried in a vacuum oven set at 120° C. for more than 12hours. LiCoO₂ was used as a counter electrode, lithium metal foil wasused as a reference electrode, and 1 mole of LiPF₆/EC:DEC (volume ratio1:1) was used as an electrolyte to prepare a beaker-shaped threeelectrode cell in a dry box under argon atmosphere.

A constant current test was conducted for the cell at room temperatureunder the following conditions. Constant current was added to 0 V (vs.Li/Li+) at a current density of 30 mAg⁻¹ to charge the cell, anddischarge was conducted to 2V (vs. Li/Li+) at a current density of 30mAg⁻¹. A 5-minute open time was set between charge/discharge. FIG. 8 isa charge/discharge graph conducted under the above conditions, and FIG.9 is a graph showing the results of reversibility. As results ofcharge/discharge, 308 mAh/g of reversible capacity appeared, initialcoulomb efficiency was 89.3% and stable reversibility appeared till 10thcharge/discharge.

Example 18

The spherical non-graphitizable carbon prepared in Example 14 as activematerial and polytetrafluoroethylene (PTFE) as a binder were mixed at aweight ratio of 10:0.5 to prepare a paste, the paste was adhered to acopper mesh current collector to prepare a working electrode, and theelectrode was dried in a vacuum oven of 120° C. for more than 12 hours.LiCoO₂ was used as a counter electrode, lithium metal foil was used as areference electrode and 1 mole LiPF₆/EC:DEC (volume ratio 1:1) was usedas an electrolyte to prepare a beaker-shaped tree electrode cell in adry box under argon atmosphere.

Constant current constant voltage test was conducted for the cell atroom temperature under the following conditions. Constant current wasadded to 0 V (vs. Li/Li+) at a current density of 30 mAg⁻¹, anddischarge was conducted to 2V (vs. Li/Li+) at a current density of 30mAg⁻¹. A 5-minute open time was set between charge/discharge. Theresults of charge/discharge were as follows: a reversible capacity of451 mAh/g appeared, an initial coulomb efficiency was 64.2%, and astable reversibility appeared until 10^(th) charge/discharge.

Example 19

Methylnaphthalene-derived anisotropic pitch (softening temperature=227°C.) was mixed with TS-530 silica, a surface of which was treated withhydrophobic inorganic substance hexamethyldisilzane, and the mixture washeat-treated at 280° C., which more than the softening temperature, for1 hour under argon atmosphere to prepare a spherical carbon precursor.FIG. 10 is photograph of the prepared carbon precursor by ScanningElectronic Microscopy with a magnifying power of 1000.

The spherical carbon obtained in the present invention has high a tapdensity and a small specific surface area compared to non-sphericalcarbon. When used for anode active material for a lithium secondarybattery, the spherical carbon of the present invention can increase thepacking density and thus can increase battery capacity per unit volume,and can decrease initial irreversible capacity because it has a smallspecific surface area. Further, spherical artificial graphite can beprepared by graphitizing spherical graphitizable carbon in a simpler andmore inexpensive process than that used for the existing g-MCMB.

1. A battery comprising the spherical non-graphitizable carboncomprising 10 wt % or more of spherical particles satisfying thefollowing Mathematical Formula 1:0.99≦a/b≦1  [Mathematical Formula 1] where a is a minor axis of aparticle and b is a major axis of a particle.
 2. The battery accordingto claim 1, wherein the remaining particles of the sphericalnon-graphitizable carbon satisfy the following Mathematical Formula 2:0.1≦a/b<0.99  [Mathematical Formula 2] where a is a minor axis of aparticle and b is a major axis of a particle.)
 3. The battery accordingto claim 1, comprising a cathode, an anode, an electrolyte, and aseparator inserted between the anode and the cathode, wherein the anodecomprises the spherical non-graphitizable carbon.
 4. The batteryaccording to claim 1, wherein the average particle diameter of thespherical non-graphitizable carbon is 1 to 40 μm.
 5. The batteryaccording to claim 1, wherein the specific surface area of the sphericalnon-graphitizable carbon is 3 m²/g or less.